The present invention relates to a method of measurement of absolute capacitance (self-capacitance) of an object in the proximity of a plurality of independent electrodes, these electrodes having non-regular surfaces. It also relates to an electronic gestural interface device implementing the method.
The field of the invention is more particularly, but non-limitatively, that of touch-sensitive and 3D capacitive surfaces used for human-machine interface commands.
Communication and work devices are increasingly using a touch-sensitive control interface such as a pad or a screen. It is possible for example to mention mobile telephones, smartphones, computers with touch-sensitive screens, pads, PCs, mouse devices, touch pads, widescreens, etc.
A large number of these interfaces use capacitive technologies.
The touch-sensitive surface is equipped with conductive electrodes connected to electronic means which make it possible to measure the variation of the capacitances appearing between electrodes and the object to be detected in order to give a command.
The capacitive techniques currently used in touch-sensitive interfaces most often use two layers of conductive electrodes in the form of rows and columns. The most widely used geometric topology is that each row and each column is composed of rhombuses which are connected together in the vertical direction to form a column and in the horizontal direction to form a row.
Two operating modes can be produced using this geometric topology for detecting the presence of an object in front of the surface:
1. The electronics measure the coupling capacitances which exist between these rows and columns. When a finger is very close to the active surface, the coupling capacitances in the proximity of the finger are modified and the electronics can thus locate the position in 2D (XY), in the plane of the active surface. These technologies make it possible to detect the presence and the position of the finger through a dielectric. They have the advantage of allowing a very good resolution in the location of one or more fingers in the plane (XY) of the sensitive surface. These techniques have however the drawback of generating in principle large leakage capacitances at the level of the electrodes and of the electronics. These leakage capacitances can moreover drift over time due to ageing, the deformation of the materials or the effect of the variation of the environmental temperature. These variations can degrade the sensitivity of the electrodes, or can even trigger commands in an untimely manner. Another drawback of this technique is that the electric field generated between the rows and the columns remains especially concentrated around the surface and the change of coupling capacitance does take place only for objects very close to the surface, or even in contact. This limits this technique to touch and 2D use exclusively.
2. The electronics measure—for each row and each column of electrodes—the absolute capacitance which appears between the object and the electrode in question. The advantage of this method is that the electric field is radiated further from the surface, making it possible to measure objects which are located several centimeters above the screen. The drawback of this method is the limitation of detecting several objects because of a positional ambiguity of two objects, in fact if the X or Y coordinates of these objects are permuted, the capacitances measured will be identical. For a person skilled in the art, this phenomenon is known as “ghosting”.
Techniques are also known that make it possible to measure the absolute capacitance which appears between electrodes and an object to be detected. For example, document FR 2 844 349 by Rozière is known, which discloses a capacitive proximity detector comprising a plurality of electrodes which will be independently excited and measured. This detector makes it possible to measure the absolute capacitance and the distance between the electrodes and the objects in the proximity.
These techniques make it possible to obtain measurements of capacitance between the electrodes and the objects with high resolution and high sensitivity. This makes it possible to detect for example a finger at a distance of several centimeters without ambiguity. The detection can be done in three-dimensional space (XYZ) but also on a surface, in a plane (XY). These techniques offer the possibility of developing truly contactless gestural interfaces and also make it possible to improve the performance of touch-sensitive interfaces.
In order to interpret the measurements easily, to detect the presence of an object reliably and to estimate its position accurately, the electrodes are ideally disposed regularly on the surface, which preferably results in disposing the electrodes in a configuration having identical rectangular geometry for all of the electrodes. The size of the electrodes is approximately identical to or smaller by about 50% than the size of the object to be detected. Typically, an electrode surface area ranging from 0.35 to 0.65 cm2 is well suited to an application of the human interface type, where the object to be detected is a human finger. This type of regular partitioning is well suited to interfaces of the virtual 2D/3D button type where the electrodes are etched on an electronic printed circuit board (PCB) and are supplied by a power supply which is located below the conductive surface layer.
However, because of the constraints related to the transparency of the surfaces in certain applications of the “smartphone” type, where the detection surface must allow the passage of a maximum amount of light coming from the display, the surface of the electrodes and its electrical connection to the electronic excitation and acquisition circuit are disposed on the same layer. The electrical connections make it possible to connect the electrodes located at the centre of the screen to the periphery of the screen and then these connections descend, if this is necessary, along the circumference of the screen. The connections on the circumference can be protected or not protected from the capacitive interference of the environment by covering them with an isolating surface and then by disposing a conductor nearby, known as a guard conductor, which is excited with the same electrical potential as that of the electrodes. If the tracks are “guarded”, they are considered as non-measuring and are not considered as part of the capacitive measurements. In the opposite case, these tracks are an integral part of the measurement. The connections which connect the central electrodes to the periphery result in each individual measurement no longer being localized in a rectangular surface. It measures a response not only when the object is above the principal surface, but as soon as the object is in the proximity of the connection track which can be at a distance from the principal measuring surface.
However, the constraint of better transparency of the screen makes it necessary to dispose the electrodes and their connections on the same surface. This makes it possible to reduce the manufacturing cost. This simplification makes it possible to have great reliability by eliminating the inter-layer connection elements.